Reversibly photochromic printing inks

ABSTRACT

Photochromic printing inks are disclosed which are in particular used for the printing of security documents. Prints are normally nearly colorless and become colored when energy irradiated, such as by ultraviolet light. This photocoloration is reversible. The printing inks contain photochromic compounds which are protected against other ink components. Methods are described to prepare the inks, to print security documents, and to detect counterfeiting.

FIELD OF THE INVENTION

This invention belongs to the field of printing inks. More particularly,it is related to printing inks for the printing of documents to givestable prints which, when exposed to energy radiation such as light,will give a reversible color change called photochromism.

DESCRIPTION OF THE PRIOR ART

Photochromism, sometimes also called phototropy, has been discovered byMarckwald in 1899. It can be defined as the property of a composition tochange its visible light absorption spectrum on exposure to energyradiation of a certain wavelength or wavelength range such asultraviolet light, capably of being absorbed by said composition. Theoriginal absorption spectrum is generally restored, after a certaintime, in the dark or on gentle heating. Already by 1904, a class ofphotochromic compounds commonly referred to as "fulgides" has beendiscovered by Stobbe [Chem. Ber. 37, 2236 (1904)].

Interesting photochromic compounds are colorless in their normal stateand become colored on exposure to activating radiation. The colorformation generally comprises a ring closure or ring opening in themolecule with the formation of conjugated double bonds; ionic mechanismsare generally involved in these reactions.

Photochromic compounds and compositions have excited increasingtechnical and commercial interest during the last 15 years. Numerousphotochromic compounds have been synthesized and published. Theapplications which have become known are methods for optical datastorage, sunglasses, copying devices, and photographic techniques suchas holography.

Several different families of photochromic materials have been reported,and a range of different colors, produced by photochromism, withparticular characteristics is available today. However, whereas the useof photochromic compounds for printing purposes has already beensuggested, no printing ink formulation has become known yet. This seemsto be due to the fact that photochromic compounds are generally highlyreactive and normally undergo chemical and photochemical changes in thecomplex medium of printing inks.

Nevertheless, attempts have been made to render photochromic compoundsmore stable in order to allow their use in printing inks. Thus, U.K.patent specification no. 1,602,755 suggests the reactions ofphotochromics of the fulgimide class, i.e. substituted bismethylenesuccinic amide derivatives, with dihalogeno or trihalogeno triazines toobtain a mono or dihalogeno triazino derivative which may be consideredas a reactive dyestuff, and the patent suggests the application of thesecompounds in the printing of security documents, including bank notes,as a precaution against forgery. However, no printing ink nor anyindication how to proceed for obtaining a printing or printed securitydocuments are disclosed.

In an analoguous but different manner, the published internationalpatent application no. WO 83/00568 describes the preparation of aparticulate photochromic material by adding a photochromic compound tofinely divided clay, kaolin, barium titanate, bentonite etc. to producea pigment. However, here again, no printing ink is disclosed in theExamples. No suggestions can be found how to print security documents.

Attempts to formulate various printing inks, using different classes ofphotochromic compounds, completely failed. The tests have shown thatphoton induced side reactions and oxidation rapidly result in the totalloss of the photochromic effect. However, there is an urgent need forprinting inks useful for printing documents to carry marks normallyinvisible to the human eye but which become visible after exposure to anactivating radiation, especially for security documents in order todetect or prevent forgery or counterfeiting.

OBJECTS OF THE INVENTION

The first and major object of this invention is to provide printing inkscontaining a reversibly photochromic compound, whose components do notinterfere with the photochromic compound and which give stablephotochromic effects.

Another important object of the invention is to provide such printinginks, formulated in view of their application in every one of thevarious printing methods, which give on conventional substratesphotochromic printings having good long-lasting stability properties.

A further object of this invention is to provide photochromic printinginks fulfilling the above described conditions wherein the photochromiccomponents have good photosensitivity, a high color-changing reactionspeed, a sharp photochromic color change and an acceptable high numberof color change cycles.

A further object of the invention is to provide photochromic printinginks especially adapted to their application in the field of securitydocuments in order to detect or prevent forgery.

A further and important object of this invention is to providephotochromic printing inks whose physical and chemical properties aswell as the behaviour on printing machines are such that these inks canbe used simultaneously with conventional printing inks or even inadmixture with these known inks, in the printing of security documentsto confer photochromic properties to them.

Another object of the invention is to provide methods of incorporatingappropriately selected photochromic compounds into said printing inksand to develop methods for the preparation of such printing inks,adapted to be used in every one of the known printing techniques.

Still a further object of this invention is to provide protectionsystems for photochromic compounds within printing inks and within theprints obtained therefrom, in order to achieve or increase theirstability against chemical, radiation and heat induced desactivation.

Another object of the invention is to provide methods for testingdocuments which have been marked with a print obtained from the printinginks of this invention.

SUMMARY OF THE INVENTION

Now, it has surprisingly been found that photochromic compounds can beused in printing inks when care is taken and special measures areapplied to protect these compounds from desactivation by oxygen anddetrimental chemical influences, in particular from ionic ones, duringthe formulation and the use of printing inks.

The above defined objects and still others are attained by the inventionwhich in its broadest aspect, provides protection of the photochromiccompounds from chemically and energy induced alterations, by embeddingthe photochromic compounds into a light-transparent, inert polymericprotecting composition being a barrier for oxygen and for components ofthe printing ink which would interact with the photochromic reactionsites of the photochromic compounds.

This embedding can be effected by incorporating the photochromiccompounds, before their blending into an ink formulation, into apolymeric composition which is then broken into lumps and subsequentlymicronized to a particle size of from 1 to 20 micrometers. Anothermethod is to encapsulate the photochromic compounds by the process ofcoacervation or encapsulation to form microcapsules, a method known perse, wherein the photochromic compound is enveloped by a thin shell of aninert light-transparent polymer. These two embedding methods result innew and useful dyestuff compositions having the general properties oforganic pigments.

An embedding of the photochromic compound into a protecting polymer canalso be achieved during and after printing when the photochromiccompound is directly incorporated into a printing ink formulation.However, bearing in mind that the photochromic compounds must beprotected, according to the invention, from any detrimental influencesimpairing the desired properties of the photochromic compounds in theprinting inks, especially long-time stability, oxygen stability, speedof color change and sharpness of color change, and selectivity ofphotochromic wavelength, the inks must be formulated such that they donot contain certain components. The inks must be devoid of any compoundsirreversibly interfering with the photochemical coloration mechanism ofthe photochromic compounds. Details will be discussed later.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, abbreviations will be used for the sake ofshortness. "Photochromic" or "photodye" means a photochromic compound."Photodye-stuff" means an organic pigment-like particulate photochromiccomposition where the photochromic is surrounded by a protectingpolymer, according to the invention. "Chemical hardening" or "chemicaldrying" means the polymerization, curing or crosslinking of printing inkbinder precursors. "Physical drying" means solidification by the mereevaporation of solvents.

Furthermore, the term "photochromism" or "photochromic condition" asused herein, is meant to define the capability to change the color,generally from colorless to any visible coloration, after exposure forat most about 5 minutes to an energy radiation source having awavelength comprised from about 200 to about 1100 nm. The change ofabsorption behavior may occur instantaneously on exposure to the idealwavelength of the particular photochromic reaction or after overcomingany activation phase.

The term "color" or "coloration" is to be understood as the overallabsorption characteristic within the same range of wavelengths of theelectromagnetic spectrum. The color must be reversible and should lastfor a time period comprised from picoseconds up to about 30 minutes.Reversibility, i.e. color cancellation, may be accelerated by heating orby exposure to a distinct wavelength radiation different from that ofthe first coloration.

The photochromics which can be used in the printing inks of thisinvention, are generally already known substances selected from threemain classes A, B, and C hereinafter described.

Class A encompasses spiro-indolino-naphthoxazines as disclosed in U.S.Pat. Nos. 3,562,172, 4,342,668, 4,440,672 and 4,699,473. In thesecompounds, basically the same photocoloration reaction is involved as inthe photochromics of class C. Class A further comprisesspirobenzopyranes as described in Japanese patent publication no.61-1004, published 13 Jan. 1986. They are chemically closely related tothe above cited photochromics of Class A and have still the samephotocoloration mechanism.

Class B comprises fulgides and fulgimides, disclosed for example in U.S.Pat. Nos. 4,182,629, 4,145,536, 4,220,708, and U.K. patents no.1,442,628 and 2,002,752. Fulgides are derivatives of bis-methylenesuccinic anhydride, and fulgimides are derivatives of bis-methylenesuccinic imide where the imide nitrogen may be substituted by alkyl,aryl or aralkyl.

Class C comprises spiro(1,8a)-dihydroindolizines, disclosed anddescribed for example in German patent publications no. DE-A-29 06 193,32 20 257, 33 20 077 and 35 21 432. The photochromic reaction involvesthe opening of a heterocyclic ring at the spiro location to form abetain-like ionic structure comprising an oxygen anion and a nitrogenkation.

Now, the invention contemplates three principal methods to introducethese photochromics into printing inks. These methods will now bedescribed more in detail.

In all methods, one or more photochromics can be used, taken from one ormore of the aforesaid classes A to C.

The first two methods, numbered as methods I and II, involve thepreparation of pigment-like photodyestuffs. These pigment-like photodyescomprise the photochromic embedded in a transparent polymer.

In method I, the photochromic of class A, B or C is incorporated into athermoplastic, light-transparent polymer by melting together the twocomponents at a temperature comprised between 50° and 150° C. Thetemperature is selected such that the photochromic is not altered oreven destroyed. Of course, the polymer has to be selected such that itsmelting point or melting range is low enough so that it will form themolten state at a temperature which does not alter the photochromic.

The melt so obtained is mixed thoroughly until perfect homogenization,and the photochromic will be uniformly distributed in the melt as asolution or dispersion.

An adequate inert solvent may be added before melting in order to lowerthe liquid temperature of the mixture.

Then, the mixture is allowed to cool to ambient temperature. Should asolvent have been used, it is stripped off during or after cooling. Thephotochromic now forms a solid solution or dispersion with the polymer.

The solid mass so obtained is then broken into lumps which are thenmicronized in a suitable equipment, e.g. the "Alpine AFG 100" mill, to aparticle size in the pigment range, i.e. within 1 and 20 μm. Theparticulate composition, after micronization, is appropriate for anintroduction into binder preparations on standard dispersion equipmentssuch as three roller mills, high-speed dissolvers, or bead mills, knownin the art of manufacturing printing inks, to form liquid or pasty inksby adding the other standard ink components, for various technicalapplications.

Such thermoplastic polymers for embedding the photochromic must furtherbe selected which are free from groups which would interfere with thephotocoloration reaction, especially free from carboxyl groups, whichwill form a barrier for oxygen, and which are inert to any othercomponent of the printing ink such as solvents or binders, in order tomaintain their protecting function for the photochromic and to ascertainthat they will not be attacked by, e.g., a solvent of the ink so thattheir pigment-like configuration is not altered.

Preferred thermoplastic resins which fulfill the above mentionedconditions, are for example cellulose derivatives, polyamides, acrylicpolymers and copolymers, polyvinylic resins, hydrocarbon resins such aspolyolefines; polyesters, polycarbonates, or mixtures thereof.

The micronized pigment-like, particulate solid solutions of thephotochromic in the thermoplastic resin, obtained in this Method I, areespecially well suited for incorporation into liquid printing inkvehicules which are unable to dissolve or swell the thermoplastic resinof the protected photochromic. The resulting photochromic inkconstitutes an aqueous or a non-aqueous dispersion of a photochromicembedded in a thermoplastic host polymer.

Method II involves the encapsulation of a photochromic in a transparentpolymer shell, in the form of microcapsules.

The microencapsulated photochromic dyestuff is prepared by encapsulationof the photochromic, selected from above described classes A, B or C, inaprotic resins or solvents according to the well known methods ofmicroencapsulating or coacervation using gelatine or acacia gum as awall material, or according to U.S. Pat. Nos. 4,428,978 and 4,517,141which are incorporated herein by reference.

According to U.S. Pat. No. 4,428,978, aqueous suspensions containing 35to 60% by weight of microcapsules having a size of from 1 to 20 μm(microns) are prepared from polyisocyanates and hydrogen activecompounds by a boundary surface polyaddition process. After polyadditionunder shearing conditions, the suspension of the microcapsules may bedried to prepare dry microcapsules. The microcapsules, comprising theenclosed photochromic, are then incorporated into printing ink vehiculesin any standard dispersing equipment, usually in simple butterfly mixersor dissolvers under limited shear rates for not damaging themicrocapsules.

Method III: It has surprisingly been found that photochromics ofaforesaid classes A, B and C can, under certain conditions, beintroduced directly into printing ink vehicles. In these inks, thephotochromics are found to be embedded, after drying of the ink, in thebinder, and are protected from alterations by oxygen or ionic species.

The conditions for the incorporating according to Method III are thefollowing: The amount and the nature of functional groups of theselected binder resin of the ink or those created during chemicalhardening, must be such that they are unable to interfere with thephotochromic reactions, i.e. ring opening and ring closure reactions,involved in photocoloration and decoloration. In particular, theformation and presence of ions are to be excluded. As solvents,hydrocarbons are preferred.

Functional groups particularly to be excluded in the molecules of theink components are carboxy groups and their salts. Ionic polymerizationinitiators such as cobalt or manganese octoates are to be excluded. Freeradical forming initiators and free radical induced chemical hardeningreactions are surprisingly not interfering with the photochromicreaction and may be permitted in the system. This is of greatestimportance since chemical hardening on the basis of free radical inducedcrosslinking or polymerization can be performed in this method, givingrapidly drying and high performance prints, which is not attainable withmere physical drying.

Any kind of polymeric resin can be used in Method III which iscompatible with the solvents being used and which has an acid numberbelow 10, expressed as mg KOH per gram of resin, generally known for theuse in offset, lithographic, letterpress and intaglio (plate printing)inks.

Examples for free radical curable resins which may be used in Method IIIare epoxyacrylates, polyester acrylates, urethane acrylates and thecorresponding monomeric compositions--where polymerization is effectedin situ--, namely TMPTA (trimethylolpropane triacrylate), PTA(pentaerytritol triacrylate) and HDDA (hexanediol diacrylate) for thepreparation of UV hardening offset, litho and letterpress inks.

Furthermore, thermoplastic polymers as mentioned for Method I may beused, as far as they respond to the above defined conditions and limitoxygen diffusion, since photochromics are known to be oxygen sensitive,such as polycarbonates, polyethers and some cellulose derivatives likecellulose acetobutyrate or acetopropionate.

The invention contemplates different printing inks. The printing methodsthe printing inks of the invention are provided for are the following:

flexographic printing,

gravure or heliogravure printing,

intaglio (steel engraving) or plate printing,

letterpress printing,

indirect letterpress or dry offset printing,

lithographic or wet offset printing, and

screen printing.

All these printing methods are well known to the man skilled in the art.It is furthermore well known that the composition and most physicalproperties of printing inks must be adapted to the respective printingmethods. For example, gravure printing inks have a much lower viscositythan intaglio printing inks. The different basic ink compositions arealso well known to the one skilled in the art and will therefore not bedescribed here in detail.

The photochromic printing inks of the invention will contain one orseveral photochromics or photochromic dyestuffs in concentrationscomprised between about 0.5 and about 50% by weight, referred to thetotal weight of vehicle, binder and photochromic. The inks of thisinvention are to be used for the printing of identification or forgerydetection marks or patterns on security documents. This term comprisesbank notes, checks, traveler's checks, stamps, shares, passports, labelsand similar printed documents for which measures against counterfeitingare indicated.

As it has already briefly been stated above, the preferred use of theprinting inks of this invention is the printing of security documents inorder to allow identification and detection of falsifications.

The inks of this invention have been developed for being usedsimultaneously with conventional printing inks for the printing ofsecurity documents, i.e. on the same machines and by simple substitutionof one of the normally used printing inks. For example, a banknote canbe designed such that normally blue lines are printed in the blue inkingapparatus of a multicolor offset printing press with a photochromicprinting ink of the invention, and these blue lines become only visibleafter exposure to U.V. light. In this manner, a counterfeited banknotewill easily be recognized. Details are given below.

The printed detection marks are normally invisible. On activation asdescribed, a reversible photochromic coloration takes place which iscapable of being detected by normal visualization. However, especiallyfor short-lasting photocoloration, this invention further contemplates adetection method. This method may be carried out in special detectiondevices and comprises the irradiation of the document under test withenergy radiation having a wavelength capable of being absorbed by thedetection mark of the document, printed with a photochromic ink andadapted to the light range of the photochromic reaction, for exampleusing an UV lamp having an average power of from 2 to 25 watts, thensensing the photocoloration and interpreting the color change, andfinally desactivation for the reversal of the photocoloration. Thesensing of the photocoloration may be effected by the human eye or byappropriate color sensitive electronic components known per se, such asphotodiodes, phototransistors, light dependent resistors, PIN diodesetc., which sense the color difference before and after the photochromicreaction. An electronic comparator, known per se too, will then give asignal responsive to the color change.

Desactivation or reversal of color may be effected by gentle warming ofthe document, by IR radiation, or by another energy irradiation whichbrings about the reverse photoreaction.

The following examples are given to allow a still better understandingof the invention but will not limit the invention thereto.

In the Examples, all parts and percentages are by weight if nototherwise indicated.

EXAMPLES 1 TO 5

Method I

Example 1. Aqueous ink for gravure and flexographic printing

Part 1

3 parts of a photochromic compound of class A above are first dissolvedin 57 parts of cellulose acetate propionate (CAP 504--0,2--Eastman), 20parts of toluene and 20 parts of ethanol by heating the mixture to 60°C. for 15 minutes.

The mixture is then dried at 100° C. for fifteen hours under reducedpressure until total evaporation of solvents. The solid solutionobtained is broken up and then micronized, and the finely dividedparticles thus obtained having a size of from 1 to 20 microns areincorporated in the following ink formulation by means of a three rollermill.

Part 2

An aqueous heliographic and flexographic ink is formulated as follows:

    ______________________________________                                                                  %                                                   ______________________________________                                        48% solids emulsion of acrylic polymers in                                                                69                                                water (Joncryl 89 - Johnson)                                                  49% solids emulsion of acrylic polymers in                                                                6                                                 water (Joncryl 74 - Johnson)                                                  Antifoaming agent (Silicone DC 3 - Dow Corning)                                                           0.02                                              Polyethylene wax (Ceridust VP 3715 - Hoechst)                                                             1.0                                               Isopropyl alcohol           7.98                                              Water                       3                                                 Solid solution of photochromic dye in                                                                     13                                                cellulose acetate proprionate, as prepared above                              ______________________________________                                    

The two emulsions of acrylic polymers are mixed with the antifoamingagent. Polyethylene wax is dispersed therein for ten minutes in a ballmill. Finally isopropyl alcohol, water and the photochromic dyestuff areincorporated into the mixture by means of a ball mill.

After conventional printing on paper and drying, the colorless printturns blue when irradiated with light having a wavelength of about 366nm.

EXAMPLE 2

Example 1 was repeated with the exception that a photochromic of Class Babove was substituted for the same amount of photochromic A.

An aqueous photochromic printing ink for gravure and flexographicprinting was obtained.

After conventional printing on paper and drying, the colorless printturns pink when irradiated with light having a wavelength of about 366nm.

EXAMPLE 3

Example 1 was repeated with the exception that a photochromic of Class Cabove was substituted for the same amount of photochromic A.

An aqueous photochromic printing ink for gravure and flexographicprinting was obtained.

After conventional printing on paper and drying, the colorless printturns green when irradiated with light having a wavelength of about 366nm.

EXAMPLE 4

Photochromic intaglio ink

Part 1 of Example 1 was repeated to prepare a solid solution of aphotochromic of class A in cellulose acetate propionate, having aconcentration of 5% by weight, and a particle size comprised between 1and 20 microns. Then, a photochromic intaglio ink for engraved steelplate printing of detection marks on bank notes was prepared by mixingthe following ingredients under gentle heating on a three roller mill:

    ______________________________________                                        Addition product of tung oil and maleic acid                                                              35%                                               modified phenolic resin in a high boiling                                     mineral oil (PKWF 28/31)                                                      Long oil alkyd resin in ink solvent 27/29                                                                 7.5%                                              (Shell Ch. Ind.)                                                              Alkylphenolic resin modified with raw tung oil                                                            16.0%                                             in ink solvent 27/29 (Shell Ch. Ind.)                                         Polyethylene wax            1.5%                                              Solid solution of photodye A in cellulose                                                                 39.7%                                             acetate propionate (Example 1, part 1)                                        Cobalt octoate (11% metal)  0.05%                                             Manganese octoate (10% metal)                                                                             0.10%                                             Lead octoate (38% metal)    0.15%                                             ______________________________________                                    

The use of photochromics of classes B and C, replacing the photodye ofclass A, resulted in other useful intaglio printing inks.

EXAMPLE 5

Photochromic offset and litho printing ink

Part 1 of Example 1 was repeated to prepare a solid solution of 5% byweight of a photochromic compound of class A above in cellulose acetatepropionate having a particle size of 1 to 20 microns.

Then, a photochromic offset and litho printing ink for the printing ofdetection marks was prepared by mixing the following ingredients:

    ______________________________________                                                                %                                                     ______________________________________                                        Phenolic resin modified rosin cooked with                                                               24                                                  linseed oil                                                                   Long oil alkyd (Alftalat AL810 -                                                                        35                                                  Reichhold Albert Chemie AG)                                                   Polyethylene wax (PE 130 - Hoechst)                                                                     2                                                   Ink solvent 27/29 (Shell Industrial Chemicals)                                                          2                                                   Cooked linseed oil        5                                                   Solid solution of photodye A in cellulose                                                               26.2                                                acetate propionate (5%) (Example 1, part 1)                                   Titanium dioxide          5                                                   Cobalt octoate (10% cobalt)                                                                             0.8                                                 ______________________________________                                    

The ink, after printing and drying, gives a blue photochromic colorationunder irradiation.

This Example was repeated except that a photochromic of class B andclass C, respectively, was replaced for the photochromic of class A.These inks, after printing and drying, resulted in pink or greenphotochromic colorations, respectively, under irradiation.

EXAMPLE 6

Photochromic intaglio printing ink Method II

Part 1: microcapsules

5 parts of a photochromic of class A are dissolved in 50 parts ofchloroform to obtain a clear solution A. Following the encapsulatingprocedure described in U.S. Pat. No. 4,517,141, solution A is addedslowly at room temperature to 270 parts of a stirred solution of 0.65parts of polyvinyl alcohol, 1.35 parts of xanthan gum, and 9 parts ofdiethyl tolylene diamine in 268 parts of distilled water. The shearingrate is increased for 1 minute resulting in the formation of a fineoil-in-water emulsion. Then, the stirrer speed is reduced, and 180 partsof a solution of 20 parts of powdered hexamethylene diisocyanate, sodiumbisulfite adduct, in 160 parts of a colloid solution containing 0.25% ofpolyvinyl alcohol and 0.50% of xanthan gum, are added to thediamine/photochromic emulsion.

Emulsification is enhanced by applying high shear rate conditions, andthe slurry obtained is transferred into a reaction vessel equipped witha reflux condenser and heated for 6 hours to about 60° C. under gentlestirring. Then, the slurry is spray dried.

Dry, transparent microcapsules are obtained having a diameter between 1and 20 micrometers and containing about 10% of the photochromic. Whenirradiated with light having a wavelength of about 366 nm, a strong bluephotochromic effect is obtained.

Part 2: printing ink

An intaglio (steel plate) printing ink is obtained by blending thefollowing ingredients under gentle warming on a three roller mill:

    ______________________________________                                        Addition product of tung oil and maleic                                                                  35%                                                acid modified phenolic resin in a high                                        boiling mineral oil                                                           Long oil alkyd resin in ink solvent                                                                      7.5%                                               27/29 (Shell)                                                                 Raw tung oil modified alkylphenol resin in                                                               16%                                                ink solvent 27/29 (Shell)                                                     Polyethylene wax           1.5%                                               Microcapsules of part 1 of this Example                                                                  35%                                                Titanium dioxide           4.7%                                               Cobalt octoate (11% metal) 0.05%                                              Manganese octoate (10% metal)                                                                            0.10%                                              Lead octoate (38% metal)   0.15%                                              ______________________________________                                    

After conventional printing on paper and drying, a nearly colorlessprint is obtained which turns blue when irradiated with light having awavelength of about 366 nm.

When the photochromic of class A in this Example is replaced in part 1by the same amount of a photochromic of classes B or C, excellentintaglio printing inks are also obtained.

EXAMPLES 7 to 18

Method III

EXAMPLE 7

Photochromic silk screen ink

A screen ink formulation are prepared from the following ingredientsusing the indicated amounts:

    ______________________________________                                                            %                                                         ______________________________________                                        Ethyl cellulose (N7-Hercules)                                                                       16                                                      Ethoxypropanol        16                                                      Shellsol A            50                                                      Calcium carbonate     8                                                       Titanium dioxide      7                                                       Photochromic of class A                                                                             3                                                       ______________________________________                                    

The ink is prepared by mixing all these ingredients together, except ofcalcium carbonate and titanium dioxide, and heating the mixture up to80° C. shortly in order to achieve a complete solution of thephotochromic material. Calcium carbonate and titanium dioxide are thenincorporated into the solution by means of a three roller mill. Thefinal printing viscosity of the ink is about 40 poises at 25° C.

After conventional printing on paper and drying, the colorless printturns blue when irradiated with light at 366 nm wavelength.

EXAMPLE 8

The same ingredients as in Example 7 are used with the same amounts forthe screen ink no 2 except that the photochromic of class A is replacedby a photochromic of class B at the same level (3%).

After conventional printing and drying, the print turns pink whenirradiated with light at 366 nm wavelength.

EXAMPLE 9

The same ingredients as in Example 7 are used at the same amounts forthe screen ink no 2 except that the photochromic of class A is replacedby a photochromic of class C at the same level (3%).

After conventional printing and drying, the print turns green whenirradiated with light at 366 nm wavelength.

EXAMPLE 10

Photochromic gravure printing ink

A gravure ink, based on a nitrocellulose resin, is prepared from thefollowing ingredients using the indicated quantities:

    ______________________________________                                                               %                                                      ______________________________________                                        Nitrocellulose (A280 - Societe Nationale des                                                           15                                                   Produits Explosifs S.N.P.E., France)                                          Dibutyl phthalate        6                                                    Ethanol                  34.5                                                 Ethoxypropanol           10                                                   Ethyl acetate            31.5                                                 Photochromic of class A  3                                                    ______________________________________                                    

All the ingredients are mixed together at 50° C. to obtain a clearsolution. The ink is adjusted to the suitable printing viscosity bymeans of adding ethyl acetate.

When the photochromic of class A in this Example is replaced by the sameamount of a photochromic of class B or C, excellent photochromic gravureprinting inks are also obtained.

EXAMPLE 11

Photochromic gravure printing ink

A gravure printing ink, based on a polyamide resin, is prepared from thefollowing ingredients using the indicated quantities:

    ______________________________________                                                              %                                                       ______________________________________                                        Polyamide (Eurelon 962 - Schering)                                                                    30                                                    Ethanol                 45                                                    Propyleneglycol methyl ether                                                                          5                                                     Ethyl acetate           17                                                    Photochromic of class A 3                                                     ______________________________________                                    

The manufacturing process of this gravure ink is the same as in Example10.

When the photochromic of class A in this Example is replaced by the sameamount of a photochromic of class B or C, excellent photochromic gravureprinting inks are also obtained.

EXAMPLE 12

Photochromic gravure printing ink

A gravure ink, based on an ethylcellulose resin, is prepared from thefollowing ingredients using the quantities indicated and applying themethod of Example 10:

    ______________________________________                                                            %                                                         ______________________________________                                        Ethyl cellulose (N7 - Hercules)                                                                     10                                                      Propyleneglycol methyl ether                                                                        10                                                      Ethanol               28                                                      Isopropyl acetate     44                                                      Dibutyl phthalate     5                                                       Photochromic of class A                                                                             3                                                       ______________________________________                                    

The final printing viscosity is adjusted by means of adding isopropylacetate.

When the photochromic of class A in this Example is replaced by the sameamount of a photochromic of class B or C, excellent photochromic gravureprinting inks are also obtained.

EXAMPLE 13

Photochromic heliographic printing ink

A heliographic ink, based on a vinyl resin, is formulated as follow:

    ______________________________________                                                                %                                                     ______________________________________                                        Polyvinyl resin (Rhodopas AS 85-15-Rhone                                                                25                                                  Poulenc)                                                                      Dibutyl phthalate         3                                                   Epoxy plasticizer (Edenol D81 - Henkel)                                                                 2                                                   Methyl ethyl ketone       35                                                  Isopropyl acetate         32                                                  Photochromic of class A   3                                                   ______________________________________                                    

The manufacturing process is the same as in Example 10.

When the photochromic of class A in this Example is replaced by the sameamount of a photochromic of class B or C, excellent photochromicheliographic printing inks are also obtained.

EXAMPLE 14

Photochromic flexographic printing ink

The ink, based on a nitrocellulose resin, is formulated as follows:

    ______________________________________                                                            %                                                         ______________________________________                                        Nitrocellulose (N7 - Hercules)                                                                      20                                                      Fatty amide (Armid O, Armour)                                                                       1.5                                                     Promotex PE 80 A      8                                                       Dibutyl phthalate     5                                                       Propyleneglycol methyl ether                                                                        18                                                      Ethanol               35.5                                                    Ethyl acetate         9                                                       Photochromic of class A                                                                             3                                                       ______________________________________                                    

All the ingredients are mixed together at 50° C. to obtain a clearsolution. The final printing viscosity is obtained by adding a suitablequantity of ethyl acetate.

When the photochromic of class A in this Example is replaced by the sameamount of a photochromic of class B or C, excellent flexographicprinting inks are also obtained.

EXAMPLE 15

Photochromic flexographic printing ink

This ink, based on a polyamide resin, is formulated as follows:

    ______________________________________                                                              %                                                       ______________________________________                                        Polyamide (Eurelon 945 - Schering)                                                                    36                                                    Inalco RL 727           9                                                     Fractionated gasoline (100-125° C.)                                                            27                                                    Isopropyl alcohol       25                                                    Photochromic of class A 3                                                     ______________________________________                                    

The manufacturing process is the same as in Example 12.

When the photochromic of class A in this Example is replaced by the sameamount of a photochromic of class B or C, excellent flexographicprinting inks are also obtained.

EXAMPLE 16

Offset printing ink

An offset printing ink is prepared from the following ingredients:

    ______________________________________                                                                  %                                                   ______________________________________                                        Long oil alkyd modified vinyltoluene,                                                                     63                                                acid number < 10 (Alsynol S41N - DSM Resins)                                  Long oil alkyd, acid number < 10                                                                          15                                                (Alftalat AL 810 - Reichhold Albert Chemie AG)                                Alkylphenolic resin cooked with raw wood                                                                  5                                                 oil in aliphatic hydrocarbons                                                 Ink solvent 27/29 (Shell Industrial Chemicals)                                                            2                                                 Titanium dioxide            12                                                Photochromic of class A     3                                                 ______________________________________                                    

The photochromic compound is mixed with the long oil alkyd and heatedfor a few seconds up to 80° C. to obtain a clear solution. This solutionis mixed with the other ingredients by means of a three roller mill.

The printing viscosity is about 10 Pa.s at 25° C.

When the photochromic of class A in this Example is replaced by the sameamount of a photochromic of class B or C, excellent photochromic offsetprinting inks are also obtained.

EXAMPLE 17

Photochromic UV curable offset printing ink

A photochromic UV curable offset printing ink is formulated as follows:

    ______________________________________                                                                %                                                     ______________________________________                                        Trimethylolpropane triacrylate (TMPTA)                                                                  20                                                  Photochromic of class A   3                                                   Epoxyacrylate             56                                                  Precipitated CaCO3        7                                                   Titanium dioxide          5                                                   Benzophenone              4                                                   Stabilizer (hydroquinone) 3                                                   Photoinitiator (Irgacure 651)                                                                           2                                                   ______________________________________                                    

First 20 parts of TMPTA are mixed together with 3 parts of aphotochromic of class A and 1.5 parts of stabilizer under gentle heating(40° C.) in order to obtain a clear solution.

The other ingredients are then added by means of a three roller mill.

After conventional printing on paper and drying, and cancellation of theblue color consecutive to U.V. irradiation during chemical drying, theprint turns blue when irradiated again with light at 366 nm wavelength.

When the photochromic of class A in this Example is replaced by the sameamount of photochromic of class B or C, excellent photochromic printinginks are also obtained.

EXAMPLE 18

Mixed yellow photochromic U.V. curable offset printing ink

A yellow photochromic U.V. curable printing ink is formulated asfollows:

    ______________________________________                                        TMPTA                     20%                                                 Photochromic of class A   3%                                                  Expoxyacrylate            56%                                                 Precipitated CaCO.sub.3   4%                                                  Yellow pigment (Irgalite BWA)                                                                           8%                                                  Benzophenone              4%                                                  Stabilizer (hydroquinone) 3%                                                  Photoinitiator (Irgacure 651)                                                                           2%                                                  ______________________________________                                    

The manufacturing procedure is the same as in example 17.

After conventional printing on paper and drying, and cancellation of thegreen color consecutive to U.V. irradiation during drying, the print isyellow. After exposure to light having a wavelength of 366 nm the yellowprint turns green.

The application of reversibly photochromic printing inks for themanufacturing of paper values such as banknotes, cheques, bonds, stamps,passports and others will now be discussed.

The principle printing processes involved in the production of the abovementioned securities are offset printing, litho (wet offset),letterpress printing and intaglio (steel engraving) or plate printing.

The ink formulations given as examples in this patent are perfectlyadapted to be used as such and in combination with conventional securityinks on traditional equipment.

A) Production of a banknote with photochromic properties on sheet fedmachines.

The first step in the production of a banknote involves 5 printing ofthe background designs on machines like Super Simultan III manufacturedby DLRG using banknote paper as specified for this purpose.

This machine is designed to print up to eight different colors on eachside of the sheet simultaneously.

The printing process can be carried out in dry offset technique or inthe litho (wet offset) process.

A photochromic offset ink as decribed in Example 16 is perfectly capableto replace any one of the eight colors in order to confer photochromicproperties to a part of the design work. The photochromic ink may beprinted in superimposition on or juxtaposition with other conventionalinks and is used to create photochromic design in form of complicatedline works, patterns or simple marks following the requirements of thesecurity document.

After drying for about one week, these offset printed sheet will beprocessed as usually on intaglio (plate) printing presses and numberingmachines to give the finished banknote.

B) Production of cheques with photochromic properties

Usually bank cheques are produced by offset printing only.

This kind of securities can be printed on a great number of differentmachinery including Simultan printing machines as mentioned above orfour-color offset printing machines as for example manufactured byRoland.

Again inks formulated as described in Example 5 or 16 may replace any ofthe conventional inks to confer photochromic properties to the product.

C) Production of banknotes bearing the photochromic properties in theintaglio ink.

In this case the offset printing is done in the traditional way as knownin the art of banknote production.

For the intaglio printing different machinery using either paper wipingor cylinder wiping (using the so called "water wiping" or"trichloroethylene wiping" processes) can be applied.

The machinery can be conceived to print on a continuous band of banknotepaper or on sheets.

As an example for such a printing machine the Super Intaglio Color IIImanufactured by DLRG (De La Rue Giori) shall be mentioned.

Usually three colors are printed on each side of the security documentin two passes.

An ink formulated following Example 4 is perfectly suitable to replaceone of the conventional inks in the case of using any of theaforementioned printing techniques in order to confer photochromicproperties to a part of the design.

The photochromic ink may be applied together with a conventional ink inthe same inking train or alone in a separated inking train.

D) Traveller cheques with photochromic properties

This kind of security documents may be manufactured by using one of thetechniques A, B or C on the appropriate printing substrate.

The Examples described above shall give a better understanding of thespecial printing process involved in manufacturing of security documentsbut shall certainly not be understood as exhaustive list ofpossibilities.

The preceding Examples show clearly the surprising and outstandingadvantages of this invention. It is evident that the practicalrealization of the invention in the context of the claimed matter may bevaried or modified according to the knowledge of the one skilled in theart without departing from the principles and the scope of thisinvention. Particularly in the field of printing, numerous modificationsand improvements are possible in formulating printing inks. However,such modifications and improvements are comprised in the scope ofprotection conferred by this invention.

What is claimed is:
 1. A method for preparing a photochromic printingink for the printing of ink accepting surfaces, comprising the followingsteps:(A) providing at least one photochromic compound and athermoplastic, light-transparent polymer not reactive with saidphotochromic compound and being a barrier for oxygen and for inkcomponents capable of irreversibly interfering with the photochromicreaction sites of said photochromic compound, (B) melting together saidphotochromic compound and said polymer at a temperature comprisedbetween 50° and 150° C. to form a liquid solution or dispersion of thephotochromic compound within the polymer, (C) allowing said liquidsolution or dispersion to cool to ambient temperature to form a solidsolution or dispersion, (D) breaking up the solid mass obtained in step(C) to lumps and micronizing the lumps to particles having a size offrom 1 to 20 micrometers, (E) incorporating the particles obtained instep (D) into a printing ink formulation and (F) collecting the obtainedink comprising a particulate polymeric binder, a liquid phase, and atleast one photochromic compound, embedded in said polymeric binder. 2.The method of claim 1 wherein an inert solvent for the polymer is addedin step (B) and stripped off in step (C).